Such a vehicular transmission, which comprises a continuously variable ratio-change mechanism and a fixed ratio rotational transmission mechanism (a rotational transmission mechanism comprising a gear train) arranged in parallel with each other, has been known (for example, refer to Japanese Patent Publication No. S63(1988)-14228 and to Japanese Laid-Open Patent Publication No. 2002-48213). For example, Japanese Patent Publication No. S63(1988)-14228 discloses a transmission that comprises a belt-type continuously variable ratio-change mechanism and a reduction gear train (a gear-type rotational transmission mechanism or a fixed ratio rotational transmission mechanism), which are disposed in parallel with each other between the input shaft and the output shaft of the transmission, the input shaft being connected to a torque converter that receives the output of an engine. In this transmission, the input shaft is provided with a first hydraulic clutch, which is used for transmitting power to the reduction gear train, and with a second hydraulic clutch, which is used for transmitting power to the belt-type continuously variable ratio-change mechanism, and the output shaft is provided with a forward/reverse-switching planetary gear train and a starting clutch (main clutch). Also, Japanese Laid-Open Patent Publication No. 2002-48213 discloses a similar transmission (refer to FIG. 1 and FIG. 2).
In such a transmission, typically, the speed change ratio of the reduction gear train is set for a speed lower than a speed at the lowest speed change ratio of the belt-type continuously variable ratio-change mechanism. When the vehicle is started, the first hydraulic clutch is engaged to transmit the rotational driving force of the engine through the reduction gear train to the drive wheels of the vehicle. After the vehicle is started, the first hydraulic clutch is released, and instead, the second hydraulic clutch is engaged to transmit the power through the belt-type continuously variable ratio-change mechanism. Thereafter, a continuously variable speed change is performed for the vehicle to travel smoothly.
Japanese Laid-Open Patent Publication No. 2002-48213 discloses a transmission whose reduction gear train comprises a low-speed gear train and a high-speed gear train, which are disposed in parallel with each other, as shown in FIG. 3. In this case, also, the speed change ratio of the low-speed gear train is set at a ratio for a speed lower than that at the lowest speed change ratio of the continuously variable ratio-change mechanism, and the speed change ratio of the high-speed gear train is set at a ratio for a speed higher than that at the highest speed change ratio of the continuously variable ratio-change mechanism. This transmission transmits the power through the low-speed gear train at the starting of the vehicle, and then it is switched to the power transmission through the continuously variable ratio-change mechanism. Thereafter, when the speed change ratio of the transmission reaches the highest speed change ratio of the continuously variable ratio-change mechanism, the operation of the transmission is switched to the power transmission through the high-speed gear train.
In the above described transmission, the low-speed gear train establishes the lowest speed change ratio of the transmission to transmit a large torque necessary for starting and accelerating the vehicle from a halt condition. In such a transmission, there is a problem of shock that may be caused from a large torque acting on the drive wheels when the transmission is shifted from its neutral mode to its in-gear mode with the first hydraulic clutch coming into engagement to transmit the rotational driving force of the engine to the drive wheels (i.e., when a so-called “in-gear” operation is implemented). Especially, when the temperature of the oil used to actuate the hydraulic clutch is low, the engaging action of the hydraulic clutch is likely to become abrupt, so the above mentioned shock becomes serious. This problem impairs the user's driving experience or the comfort of the vehicle.